Color paste for manufacturing internally printed laminated glass panes

ABSTRACT

The invention relates to an improved color paste for the manufacture of internally printed, curved laminated glass panes. 
     Prior art color pastes cause problems with respect to adhesion and optical accuracy or require a higher technical complexity during the baking process for producing internally printed laminated glass panes. 
     Color pastes which constitute the invention and overcome the previous drawbacks are characterized by a print medium which contains 5,6,8-trihydroxy-1,4-naphthoquinone in addition to solvents.

The invention relates to an improved color paste for use in themanufacture of internally printed, curved laminated glass panes, as usedespecially for motor vehicles. Other objects of the invention relate toa print medium, which is contained in the color paste, and theapplication of the color paste for manufacturing internally printed,curved laminated glass panes.

Laminated glass panes, in particular those for automobiles, comprise twoglass panes, viz. an internal pane and an external pane and a plasticsheet inbetween, for example, one made of polyvinyl butyrate. Singlycurved or doubly curved laminated glass panes are used especially forthe windshield and rear windows or backlights of motor vehicles. A paintfilm in the form of an enamel is applied on places provided for thispurpose on at least one of the panes. This paint film serves to coverglue spots or internal components of the vehicle, such as attachments ofantennas and window heaters. It is expected from the paint film that itis adequately contoured and provides adequate coverage and exhibits thedesired tint. The paint film must survive the service life of thelaminated glass pane and it may not change nor peel from the glass pane.

To produce the paint film, a color paste is printed by a well-knownmethod on the intended spots of a pane and is baked in a suitablemanners. The color paste comprises in essence as colorant, inorganiccomponents, such as inorganic pigments and/or color frits; a flux, whichcan also be a mixture of several fluxes, and a medium, which is usuallycalled the print medium with respect to the most current application.Conventional print mediums comprise as essential components one or moresolvents, a polymeric organic binder and diverse processing agents. Theflux serves to fix the coloring components on the surface to bedecorated. Flux, coloring components and print medium are intensivelyhomogenized into a mixture such that there results a paste that can beprinted and is easy to bake. The color paste for producing internallyprinted laminated glass panes is normally applied by screen printing.Following printing, the solvent is removed in a drying process.Subsequently other prints can be applied, for example, a print usingso-called silver pastes to produce an electric circuit. Following thedrying process, firing takes place in a furnace, where organiccomponents of the color paste are burned and the vitrifying componentsare melted out and the pigment is fixed on the glass.

To produce curved laminated glass panes, the internal and external panesare laid on top of each other, heated to deformation temperature and thepair is curved using a bending template. The curved panes of the pairare then separated and, after a plastic sheet is embedded, compressedwhile heating. The described process cannot be carried outsatisfactorily without additional measures, if one of the glass paneswas coated with an enamel paint.

JP-A 62-72548 is based on a process for producing internally printedcurved laminated glass panes. After imprinting a glass pane with aconventional enamel paste, the solvent is removed by drying andsubsequently the glass pane that is thus decorated is fired withoutdeformation at baking temperatures around 550° C., until no more carbonis left in the paint. Subsequently the second pane is laid on theimprinted pane and the pair is curved in the furnace at a deformationtemperature of from above 550° up to about 650° C. The drawback withthis process is that the pre-firing of the glass can produce opticaldistortions and a second firing is quite expensive. If one dispenseswith the firing of the imprinted first pane and fires directly theinternally printed pair, the oxygen required to burn the binder can nolonger be supplied in sufficient amounts to the enamel, so that theresult is reductive carbon, which reduces the metal oxide in the enamelpaint and thus results in a poor adhesion of the enamel on the glass andalso results in an undesired discoloration of the enamel.

In EP-A 0 535 474 the aforementioned drawback is circumvented by using aspecial enamel paint. This enamel paint contains an inorganic binderfrom the group of silicic-acid esters and preferably additionallyinorganic oxygen donators, such as lead dioxide and manganese dioxide.The imprinted pane is dried at temperatures around 300° C. Subsequentlythe enamel is baked in the laminated pair while simultaneously deformingthe laminated glass panes. The drawback with this process is that thecolor paste to be used exhibits only a low stability in storage; and inaddition, following the imprinting, a drying step is necessary at atemperature usually around 250° C.

The object of the present invention is to provide a ceramic color paste,which is adequately stable in storage, does not require any complicatedpre-drying of the imprinted pane and can be baked while the laminatedpair is being deformed.

There has been found a color paste for producing internally printed,curved laminated glass panes, containing a flux; as colorant, inorganiccomponents from the group of pigments and/or color frits that are stableto baking; and an organic solvent-containing print medium, characterizedin that the print medium contains 5,6,8-trihydroxy-1,4-naphthoquinone ina quantity of 5 to 70% by weight.

The color paste contains as essential components 40 to 90% by weight offlux, which can also be a mixture of several fluxes; 5 to 60% by weight,in particular 15 to 40% by weight, of one or several pigments; and 5 to60% by weight of a print medium and exhibits a printable consistency."Printable" in this respect means that the color paste can be usedwithout any problems when employing the conventional screen printingmethods. Besides the aforementioned main components, the color paste canalso contain conventional processing aids, for example viscosityregulators, in small quantities, in general less than altogether 5% byweight. The expert will adjust the optimal mixing ratio of thecomponents of the color paste as a function of the desired colorproperties and working properties of the printed glass pane.

Suitable fluxes are in particular glass frits. Preferably such frits areused that can be baked at the deformation temperature of the laminatedglass pane, thus between about 500° and 700° C. The glass frits can bein part or totally the so-called color frits, that is, those thatcontain coloring components. Suitable pigments are the customaryinorganic pigments, which are stable under the baking conditions.

The print medium contained in the color paste contains the componentaccording to the invention, viz., 5,6,8-trihydroxy-1,4-naphthoquinone,in a quantity ranging from 5 to 70% by weight, preferably from 10 to 50%by weight and especially 15 to 50% by weight. Furthermore, the printmedium contains one or more organic solvents in a quantity ranging from40 to 90% by weight, preferably 50 to 80% by weight. Organic solventsare adequately polar solvents which can clearly dissolve the5,6,8-trihydroxy-1,4-naphthoquinone in the requisite amount. Suitableare, for example, solvents from the group of aromatic hydrocarbons,esters, glycols, glycol ethers, glycol esters and cycloaliphaticalcohols, such as terpene alcohols. Quite suitable are also glycol etheracetates, such as monobutyl ether monoacetate. In addition to thesolvent and the naphthoquinone according to the invention, the printmedium can contain up to 5% by weight, preferably up to 3% by weight, ofone or more polymeric, organic binders. The polymeric binders serve toincrease the handling properties of the enamel that is not yet baked.Suitable binders can be chosen from the group of polyacrylates,polymethacrylates, cellulose ethers and cellulose esters, polyvinylbutyrate and alkyd resins and urethane resins. Provided a polymericbinder is present, its content in the print medium ranges in generalfrom 0.1 to 3% by weight, preferably from 0.1 to 2% by weight.

It has been found that the 5,6,8-trihydroxy-1,4-naphthoquinone, which isessential to the invention forms a print medium, which is quite suitablefor producing color pastes, in the presence of a solvent. Saidnaphthoquinone is characterized in that it volatilizes both under airand under inert atmosphere up to about 290° C. It is not ruled out thatthis volatilization is coupled with a decomposition. Surprisingly saidnaphthoquinone exerts an adequate binding and/or antisedimentationeffect, so that the color pastes produced thus are stable in storage.The volatilizability, which is possible even in the absence of oxygen,makes this substance especially suitable as an essential component inthe color pastes and print mediums according to the invention. Since nooxygen is necessary for volatilizing said naphthoquinone, the problemswhich existed to date while firing internally imprinted laminated glasspanes also are eliminated with the use of the color pastes and printmediums according to the invention. In contrast to the prior art colorpastes, the use of the color pastes according to the invention resultsin a trouble-free burning, whereby no discoloration can be determined,no cracks or hole formation occurs and good adhesion on the glass paneis obtained.

Another special advantage of the color pastes according to the inventionlies in the fact that the glass pane imprinted with this color paste canbe dried under conventional production conditions, thus at a temperaturein general below 150° C.

EXAMPLE 1 Preparation of the Print Mediums

Solvent and 5,6,8-trihydroxy-1,4-naphthoquinone (THNQ) are mixed, duringwhich process the latter dissolves. Provided binders are added, saidbinders are dissolved in the solvent prior to the addition of the THNQ.The table shows the composition of the print medium (data in % byweight).

    ______________________________________                                                    A      B     C        D    E                                      ______________________________________                                        THNQ          50       30    20     19   19.5                                 Butylcarbitolacetate                                                                        50       70    80                                               Pine Oil                            80   80                                   Ethyl Cellulose                     1.0                                       Polymethylmethacrylate                   0.5                                  ______________________________________                                    

EXAMPLES 2 TO 6

Color pastes: The preparation is according to the well-known method in athree roll mill. A lead borosilicate is used as the glass frit; a copperchrome spinel is used as the pigment; the color frit contained as thecoloring oxides Fe₂ O₃, MnO, CaO. The table shows the composition (in %by weight).

    ______________________________________                                                 Components                                                                    2       3     4         5   6                                        ______________________________________                                        Medium 1A  15                                                                 Medium 1B            19                                                       Medium 1C                  15                                                 Medium 1D                          15                                         Medium 1E                              15                                     Pigment    21        41    25      30  35                                     Glass Frit 64        40    60          50                                     Color Frit                         55                                         ______________________________________                                    

The color pastes were printed by means of screen printing; the solventwas removed by evaporation at 135° C. Following application of thesecond layer (sodium sulfate) that was provided in the known manner witha parting agent, the laminated glass pane was baked at 610° C. None ofthe paint enamel showed any formation of bubbles, holes, or cracks oroptical distortions or discolorations; all of them showed good adhesionand adequate handling.

What is claimed is:
 1. In a color paste for use in the manufacture ofinternally printed, curved laminated glass panes, comprising a flux; ascolorant an inorganic component selected from the group consisting of apigment, a color frit and mixtures thereof, which are stable to baking;and an organic solvent-containing print medium, the improvement whereinthe print medium contains 5,6,8-trihydroxy-1,4-naphthoquinone in anamount of 5 to 70% by weight.
 2. A color paste as claimed in claim 1,which contains 40 to 90% by weight of flux, 5 to 60% by weight ofpigments, and 5 to 60% by weight of a print medium and exhibits aprintable consistency.
 3. A color paste as claimed in claim 1 or claim2, wherein the print medium contains 10 to 60% by weight of5,6,8-trihydroxy-1,4-naphthoquinone, 40 to 90% by weight of at least oneorganic solvent, and 0 to 5% by weight of a polymeric organic binder. 4.A color paste as claimed in claim 1 or claim 2, wherein the solvent ofthe print medium is selected from the group consisting of terpenealcohols, glycol ether acetates, and mixtures thereof.
 5. A color pasteas claimed in claim 3, wherein the solvent of the print medium isselected from the group consisting of terpene alcohols, glycol etheracetates, and mixtures thereof.
 6. A color paste as claimed in claim 1or claim 2, wherein the print medium contains 1 to 3% by weight of oneor more binders selected from the group consisting of polyacrylates,polymethacrylates, cellulose ether and cellulose ester, and polyvinylbutyrates.
 7. A color paste as claimed in claim 3, wherein the printmedium contains 1 to 3% by weight of one or more binders selected fromthe group consisting of polyacrylates, polymethacrylates, celluloseether and cellulose ester, and polyvinyl butyrates.
 8. A color paste asclaimed in claim 4, wherein the print medium contains 1 to 3% by weightof one or more binders selected from the group consisting ofpolyacrylates, polymethacrylates, cellulose ether and cellulose ester,and polyvinyl butyrates.
 9. In a print medium for use in preparing acolor paste, said print medium comprising an organic solvent, theimprovement wherein the print medium contains5,6,8-trihydroxy-1,4-naphthoquinone in an amount of 5 to 70% by weight.10. A print medium as claimed in claim 9, comprising 10 to 60% by weightof 5,6,8-trihydroxy-1,4-naphthoquinone, 40 to 90% by weight of at leastone organic solvent for the 5,6,8-trihydroxy-1,4-naphthoquinone, and 0to 5% by weight of at least one polymeric organic binder.